Calderas are very important land forms, not only due to the importance of their ash flows in the geologic chronology and climate of the region but also due to their local control of the surface features. They also have interesting life cycles which are widely varied dependent on rock composition, tectonics and location. Lets now examine calderas: their formation, differences, and their locations on the planets surface.

Before the processes, and stages of caldera formation are described, the term caldera must first be defined. A caldera is a more or less circular volcanic depression, which is presumably formed by the collapse of an underlying magma chamber. ( Hyndman p. 265) Calderas are further subdivided into two types, explosive and subdivide. (Summer field p. 118) The size of a caldera is variable, but it is generally larger than a volcanic c

The final, most complex, type of caldera is the subsidence calderas that form in the epicontinental and marginal zones of an orogenetic belt. These generally form in pyroclastic parent rocks and produce sialic ignibrites and lava flows during their eruptions (Hyndman p. 275). The origin of the intrusions that form these calderas can be extrapolated by the examination of their extrusive products. Epicontinental andesites are derived from the partial melting of a subducting oceanic plate under the continent. The shallower the dip of the plates subduction, the further inland plutons of this composition can be formed. Dacites and rhyolotes are derived from partial melting of the continental crust, generally through the contact melting of a basaltic intrusion rising in a zone of extension. Differentiation in an andesitic or intermediate composition pluton may also be the source of these acidic rocks (Hyndman p. 286). The composition of the pluton is important, not so much in the formation of the caldera but in the type of pyroclastic and ignibrite flows that will be produced during the caldera eruptions.

Phase four of the growth of the caldera is an intermediate phase of erosion and deposition. The caldera walls erode in talus slides, and alluvial fans fill the caldera rim. Smaller eruptions and lava flows add to the filling in of the depression. Lakes often fill the caldera (One can see this occurrence in the famous Mt. Mazama, Crater lake of Oregon. (see photos)) and if this occurs, lacuestrine deposits are often found. Eventually the lake will overflow and the waters will breach the caldera walls, once again allowing the erosional processes to take over again. Denudation of the caldera's rim ultimately buries the ring fractures and back cuts the caldera walls through slope retreat. As a result, the ring fractures are often found well within the topographic expression of the caldera rim.

The second phase of the caldera formation is the eruption of major ash flows from extensional ring fractures. This may or may not be concurrent with the third phase, which is the collapse of the magma chamber roof to form the caldera. The collapse occurs along down faulting blocks an

Some common words found in the essay are:
, Mazama Crater, magma chamber, lava flows, ring fractures, subsidence calderas, land forms, phase seven, calderas form, explosive calderas, caldera walls, magma chamber hyndman, extensional ring fractures, derived partial melting, filling caldera, explosive calderas formed, result magma chamber,
Approximate Word count = 1472
Approximate Pages = 6 (250 words per page double spaced)